1 Field of the Invention
The present invention relates to an electret fiber sheet and a method of producing it.
2 Description of the Prior Art
Known electrets are described in U.S. Pat. Nos. 3998916 and 4215682. The former disclosed a web formed from a fiber prepared by cutting an electret film, while the latter discloses a web prepared by forming an electret fiber during spinning and collecting the same on a conveyer.
Both of these known electrets have random orientation of polarized charges in the sheet, since they are formed into a web by dispersing electret fibers with random direction of polarization. Thus the charges are mutually weakened or offset disadvantageously. Therefore, the electric field strength of such an electret is low, and the charges therein disadvantageously are weakened with the lapse of time.
On the other hand, a method of making electret fiber sheet by applying high voltage to a fiber sheet with contact type voltage-application electrode is described in JA-B-4433/74, however in this method the amount of polarized charge is not large.
The present invention aims at providing an electret having an excellent stability which cannot be expected from conventional electrets and an ability to retain high charges over a long period of time.